Graphflow for Rust

[yuchen-ecnu]

Primary work：Build a graphflow database for rust base on the storage of rust_graph_lib.
File structure:

1. All codes are in folder multi_graph
2. plan folder contains the operators and structure of query_plan
3. planner folder contains the catalog for planner and query_planners
4. query folder contains a structure for storing query_graphs
5. runner folder contains the executor for catalog generation and optimizer execution
6. utils folder contains some set utils.

Note：

1. We used enumeration to implement inherit and Rc<RefCell<Op>> to implement list structure of operators.
2. We used unsafe block to get operator‘s pointer to realize mutable reference under a mutable borrowed object. It may be safe in a single threaded environment now, but it also needs to be replaced by an appropriate solution.
3. Query Planner and Optimizer Executor are all running under a single threaded environment.
4. Graphflow for rust is only based on the storage of the current project.